NATURAL SOURCES (also called background
levels) in the U.S. add up to about 100 mrems/year.
Even without
human activities, these sources would still be present. They include
the following sub-categories.

1.
Terrestrial
sources
include all land based sources. U.S. community levels range from
about 15 to 100 mrem/year. Globally, levels can be as high as 2
rems/year (e.g., parts of Brazil). Various materials contribute
radiation, including granite, coal (especially in the western U.S.),
and clay (brick homes have typically twice the radiation of wood
homes). The most serious radiation exposure from terrestrial sources
is usually radon, because it is a gas (i.e., inhalation exposure).

1.
Medical
procedures
are the major source, with a single chest X-ray contributing an
average 200 mrem. Dental x-rays are far less than chest x-rays.

2.Nuclear power
plants
contribute, on average, about 1 mrem/year. However, this is only for
residents closest to the facility. Of course, this does not include
other concerns such as nuclear accidents and nuclear waste
disposal.

Types of ionizing radiation:
Direct(all of the examples in this category are charged particles)

1. alpha particles: two protons and two neutrons (equivalent to Helium atoms stripped of their electrons);
alpha particles have low penetrating power but very strong ionizing power.
In the diagram below, Americium-241 decays into Neptunium-237 ( a net
loss of 4 in atomic weight) -- the net loss is due to the release of an alpha particle
(made of 2 protons and 2 neutrons).

2. beta particles: electrons travelling at high speed outside of their atomic orbitals
beta particles have moderate penetrating power and moderate ionizing power.
In the diagram below, we see tritium (an isotope of Hydrogen made of 1 proton and
2 neutrons) decaying into Helium (made of 2 protons and 1 neutron). In this
radioactive decay, one of the neutrons has converted to a proton and released a
high speed electon (a neutron contains both a proton and electron).

3. other charged particles: single protons, various charged fragments, etc.

Indirect (these particles are not charged, but they can ionize indirectly)